Pigments in industrial processes are often used not in the form of dry powders, since such powders form dust, which leads to increased requirements in terms of workplace safety. In addition, the incorporation of powders into plastics, basecoat systems, etc., is frequently observed to be accompanied by agglomeration of the pigment powder. Homogeneous distribution of the pigment in the respective matrix is, however, difficult if not impossible to achieve.
Instead of the pigment powder, non-dusting pigment preparations are used, as known from DE-C 36 27 329 and DE-A 41 39 993. These are readily dispersible pearl luster pigment preparations, in which a platelet-shaped pearl luster pigment is coated with a polymer resin with or without plasticizers, solvents and surfactants. DE-A 41 39 993 discloses pigmented free-flowing powders with a low moisture content.
Pastes and dry preparations prepared from them, as known, for example, from EP 0 803 552, are an industrial alternative to the dry or moistened powders provided that they meet the following general conditions:
flowable consistency PA1 minimal dilatency PA1 at maximum pigment content PA1 can be used in aqueous and organic coating systems, PA1 exhibit high stability, and PA1 are characterized by a high level of compatibilty with the other components of the coating system. PA1 at least 50% by weight of one or more effect pigments, PA1 0.1-50% by weight of plasticizer or a mixture of plasticizer/nitrocellulose, and PA1 0-49.9% by weight of a solvent or solvent mixture. PA1 good compatibility with the other components of aqueous or organic coating systems, PA1 a high level of freedom from dust, PA1 a high stability PA1 good processability and PA1 very little or no nucleation tendency
The components of the preparation/paste should be chosen such that the preparation/paste is tailored to suit the other components of the respective coating system and, once incorporated, lends itself readily to homogeneous distribution.
In addition to good compatibility with the other components of the coating system, pigment preparations are required to exhibit high stability; that is, they must not tend towards phase separation. This requirement is particularly important in the case of pigment preparations based on platelet-shaped pigments, since owing to their structure such pigments have a tendency to undergo caking on phase separation and are difficult to reagitate. Homogeneous distribution of the pigment in the respective matrix is difficult, if not impossible, to achieve. For example, an aqueous pigment preparation obtained by forming an aqueous paste from a powder comprising platelet-shaped pearl luster pigments exhibits inadequate stability.
Coating compositions, such as varnishes, paints, printing inks, etc., comprising pigments based on platelet-shaped substrates are subject to handling problems insofar as the pigments, owing to their size and density, settle readily and may then become compacted to form an extremely solid sediment cake. This cake can generally be reagitated only with very great difficulty. This situation affects in particular the storage of varnishes, paints and printing inks and their processing.
Thus, numerous methods have been developed, inter alia, to solve the problem of the incorporation and handling of platelet-shaped pigments in coating compositions.
DE 36 27 329 and the EPs 0 306 056 and 0 268 918 disclose that modified platelet-shaped substrates with a polymer coating and, respectively, following treating with coupling reagents such as organotitanates, or organosilanes in covering compositions, exhibit improved settling and reagitation characteristics.
Furthermore, reagitation can be facilitated by treating the coating compositions with additives which bring about controlled flocculation (house of cards effect), pseudoplastic and/or thixotropic behavior, steric repulsion and/or electrostatic repulsion of the pigments.
Additives with thixotropic characteristics are described in EP 0 198 519 and DE-A 18 05 693. DE-A 39 22 178 discloses that mixing a suspension of a platelet-shaped substrate with spherical particles such as SiO.sub.2, TiO.sub.2 and ZrO.sub.2, for example, produces deagglomerated and readily dispersible pigments.
In EP 0 523 357, pearl luster pigments are treated with diurea derivatives, while EP 0 515 928 discloses the surface modification of pigments with polyacrylates. EP 0 650 144 discloses the coating of pearl luster pigments with SiO.sub.2 /Al.sub.2 O.sub.3 and SiO.sub.2 /ZrO.sub.2.
The pigment preparations that are based on platelet-shaped pigments and have been developed to date for use in aqueous and organic coating systems often fail to comply to a sufficient extent with the requirements as described.